U.S. patent number 9,722,706 [Application Number 14/281,561] was granted by the patent office on 2017-08-01 for multiple wavelength light-source with tracking multiplexer.
This patent grant is currently assigned to KAIAM CORP.. The grantee listed for this patent is Kaiam Corp.. Invention is credited to Henk Bulthuis, John Heanue, Alice Liu, Bardia Pezeshki.
United States Patent |
9,722,706 |
Liu , et al. |
August 1, 2017 |
**Please see images for:
( Certificate of Correction ) ** |
Multiple wavelength light-source with tracking multiplexer
Abstract
A transmitter assembly incorporating multiple laser diodes that
are wavelength multiplexed together using a planar lightwave
circuit, and where the multiplexer's transmission spectrum depends
on temperature at the same rate as the laser diodes. This allows a
design for lower loss in the multiplexer and therefore is more
power efficient.
Inventors: |
Liu; Alice (Newark, CA),
Pezeshki; Bardia (Menlo Park, CA), Heanue; John (Boston,
MA), Bulthuis; Henk (Newark, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kaiam Corp. |
Newark |
CA |
US |
|
|
Assignee: |
KAIAM CORP. (Newark,
CA)
|
Family
ID: |
59382680 |
Appl.
No.: |
14/281,561 |
Filed: |
May 19, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61824869 |
May 17, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B
10/506 (20130101); G02B 6/12019 (20130101); G02B
6/12011 (20130101); G02B 6/12009 (20130101); G02B
6/12014 (20130101) |
Current International
Class: |
H04J
14/02 (20060101); H04B 10/50 (20130101); G02B
6/12 (20060101) |
Field of
Search: |
;398/87,79,82 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Shin Kamei et al. "Recent progress on athermal AWG wavelength
multiplexer", Active and Passive Optical Components for WDM
Communications V, Proc. of SPIE vol. 6014, 60140H1-9, (2005). cited
by applicant.
|
Primary Examiner: Vanderpuye; Ken N
Assistant Examiner: Alagheband; Abbas H
Attorney, Agent or Firm: Klein, O'Neill & Singh, LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of the filing of U.S.
Provisional Patent Application No. 61/824,869, filed on May 17,
2013, the disclosure of which is incorporated by reference herein.
Claims
What is claimed is:
1. A transmitter for a wavelength division multiplexing
communication system, comprising: a plurality of laser light
sources which output light, each of the laser light sources
outputting light about different wavelengths, the wavelengths
shifting with variation of temperature of the laser light sources;
a planar lightwave circuit positioned to receive light from the
laser light sources and combine the light, the planar lightwave
circuit having a passband with a center wavelength that shifts with
variation of temperature of the planar lightwave circuit, a shift
in center wavelength with variation of temperature substantially
matching half of a shift in wavelength of the light from the lasers
with variation of temperature.
2. The transmitter of claim 1, wherein the planar lightwave circuit
comprises an arrayed waveguide grating (AWG).
3. The transmitter of claim 2, wherein the AWG includes a polymeric
material in a grating region of the AWG.
4. The transmitter of claim 3, wherein the polymeric material fills
an etched trench in the grating region.
5. A transmitter for a wavelength division multiplexing
communication system, comprising: a plurality of laser light
sources which output light, each of the laser light sources
outputting light about different wavelengths, the wavelengths
shifting with variation of temperature of the laser light sources;
a planar lightwave circuit positioned to receive light from the
laser light sources and combine the light, the planar lightwave
circuit having a passband with a center wavelength that shifts with
variation of temperature of the planar lightwave circuit, a shift
in center wavelength with variation of temperature substantially
matching thirty percent of a shift in wavelength of the light from
the lasers with variation of temperature.
6. The transmitter of claim 5, wherein the planar lightwave circuit
comprises an arrayed waveguide grating (AWG).
7. The transmitter of claim 6, wherein the AWG includes a polymeric
material in a grating region of the AWG.
8. The transmitter of claim 7, wherein the polymeric material fills
an etched trench in the grating region.
Description
BACKGROUND OF THE INVENTION
The present application relates to the field of fiber optic
communication and, more particularly, to optical packaging
techniques and designs used for multiple wavelength
transmitters.
In the past few decades, optics has gradually become the favored
media for transmitting high bandwidths of information. Compared to
electrical cabling, fiber optics can transmit modulated light for
extreme distances with low loss and low distortion. As the
bandwidth requirements in datacenters and between switches and
routers have increased, optical links are becoming necessary in
ever shorter domains. Thus gradually optics has migrated from long
haul, to metro, and now to enterprise and datacenters. In previous
decades the signal bandwidth through a fiber has increased
generally by modulating the lasers faster and having higher speed
photodetectors on the receiver. Thus the industry went from 622
Mbits/second to 2.5 Gb/s and then 10 Gb/s. But now it is becoming
harder to have the direct line rate exceed 10 Gb/s or 25 Gb/s. Thus
to get to higher speeds, it is generally necessary to put parallel
channels within the same fiber, where 40 Gb/s, for example, is
achieved using four lanes of 10 Gb/s.
This parallelism can be achieved in a number of ways. Most simply,
one could use a ribbon fiber, where there is 10 Gb/s modulated
light in each fiber. Alternatively, one could use a more advanced
modulation scheme, where the signal has multiple levels, or is
modulated in phase as well as amplitude thus achieving multiple
bits per symbol. Perhaps the most practical way is to use multiple
wavelengths of light, with each signal modulating a light beam of a
different wavelength. Because the intrinsic bandwidth of an optical
fiber is very high, all the different wavelengths can be
multiplexed with a dispersive element such as a diffraction grating
into a single fiber. At the receiver end, the wavelengths are
demultiplexed and received separately using another matching
grating and a photodiode array. Thus 40 Gb/s can be transmitted in
four lanes of 10 Gb/s each, at four different wavelengths.
This Wavelength Division Multiplexing (WDM) approach has already
been in use extensively in long haul or metro optical links.
Typically 40 or 80 channels are multiplexed into one fiber. The
problem with using this same technique for shorter distances is
that the temperature of the lasers and the multiplexer must be
accurately controlled as the optical wavelength of a laser and a
multiplexer are both temperature-dependent. Typically in a
semiconductor laser, the wavelength of generated light varies at
about 0.1 nm per degree Centigrade. The optical passband of a
wavelength multiplexer also varies with temperature, but at a
slower rate of about 0.01 nm per degree Centigrade. To have 40 or
80 wavelengths all in the same fiber, within the 30 nm range than
can be easily amplified using conventional erbium-doped fiber
amplifiers, the wavelengths have to be closely spaced at 100 GHz
(0.8 nm) or 50 GHz (0.4 nm) spacing. As the equipment temperature
varies from -5 C to 75 C, without temperature control a laser would
change wavelengths by 8 nm, and a multiplexer by 0.8 nm, in both
cases enough to run over other channels. Thus all the optical
components are carefully temperature controlled, either with
heaters or thermoelectric Peltier coolers.
An alternative for smaller distance optical interconnects that
eliminates the precise temperature control is to spread out the
wavelength range beyond the 30 nm of an optical fiber span, reduce
the number of channels, and also dramatically increase the
wavelength spacing between lasers. For example, for 40 Gb/s
applications, four 10 Gb/s channels are used over a 60 nm span,
with wavelengths at 1270 nm, 1290 nm, 1310 nm, and 1330 nm. With 20
nm spacing, even if the output wavelength of the laser moves by 8
nm, it will not run over adjacent channels. The shift of the output
wavelength of the multiplexer of 0.8 nm is inconsequential, so no
cooling is necessary. However, one still has misalignment between
the output wavelengths of the lasers and the passband center
frequencies of the multiplexer. If the wavelengths of the laser
output and the multiplexer passband center frequency are aligned at
the midpoint of the temperature range, than at the low end of the
temperature range, the laser wavelength is too short by 3.6 nm, and
at the high end of the temperature range, the laser wavelength is
too long by 3.6 nm.
To account for this variation of wavelength with temperature,
multiplexers with semi-Gaussian or flat-topped passbands may be
used, but such multiplexers tend to have increased insertion loss
for passbands covering an appreciable portion of the wavelengths of
a channel. For example, in practical implementations, the passband
wavelength of the multiplexer may be "flat-topped," allowing good
multiplexing across a 2.times.3.6 nm or 7.2 nm temperature range.
Unfortunately, when one fabricates a flat-topped multiplexer that
goes from single mode inputs to a single mode output, the insertion
loss is much higher than compared to a standard Gaussian
multiplexer. Flat-topped multiplexers, while having a widened
passband, therefore induce additional loss, which makes the
transmitter inefficient and increases power consumption.
BRIEF SUMMARY OF THE INVENTION
Aspects of the invention provide a plurality of lasers coupled with
a multiplexer having a temperature dependent passband wavelength
shift matched to laser temperature dependent output wavelength
shift. In some embodiments the multiplexer is of a "superthermal"
design, with passband characteristics that change much more with
temperature. This matches the wavelength drift with temperature of
the multiplexer passband with the wavelength drift with temperature
of the laser output, such that the wavelengths of the light from
the light sources and the multiplexer passband vary together. This
allows the use of a "Gaussian" rather than a "flat-topped" design
in the grating multiplexer that is of much lower loss.
In some embodiments wavelengths of both the laser and the
multiplexer passband vary together with temperature. In some
embodiments the lasers and multiplexer output are not the subject
of temperature control. In some embodiments an optional receiver
that tracks the variation in wavelength, or in some embodiments
simply allows for the variation, thus allows more channels, and
many more channels in some embodiments, to be used at closer
spacing, thus increasing the total bandwidth of the link.
One aspect of the invention provides a transmitter for a wavelength
division multiplexing communication system, comprising: a plurality
of laser light sources which output light, each of the laser light
sources outputting light about different wavelengths, the
wavelengths shifting with variation of temperature of the laser
light sources; a planar lightwave circuit positioned to receive
light from the laser light sources and combine the light, the
planar lightwave circuit having a passband with a center wavelength
that shifts with variation of temperature of the planar lightwave
circuit, the shift in center wavelength with variation of
temperature substantially matching half of the shift in wavelength
of the light from the lasers with variation of temperature.
These and other aspects of the invention are more fully
comprehended upon review of this disclosure.
BRIEF DESCRIPTION OF THE FIGURES
Aspects of the invention are illustrated by way of examples.
FIG. 1 shows a design for a 4.times.10 Gb/s multiwavelength
source.
FIG. 2 shows a design for a further multiwavelength source, showing
the AWG with a temperature adjustment section.
FIG. 3 shows aspects of designs for a "superthermal" multiplexer,
where the region of the insert is modified to increase the thermal
shift to match that of the laser diodes.
FIG. 4 shows the passband of a flat-top multiplexer and a Gaussian
multiplexer.
FIG. 5 shows the passband characteristics of an embodiment of the
invention, where a lower loss Gaussian design multiplexer tracks
the wavelength drift of the laser and thereby provides lower loss
multiplexing compared to the conventional flat top passband.
FIG. 6 shows an alternate design of the multiplexer where the
region of the insert is not in the arms, but in the star region of
the device.
FIG. 7 shows a further alternate design of the multiplexer.
DETAILED DESCRIPTION
Multiwavelength links generally have multiple light sources
packaged with a multiplexer that combines light from these sources
into a single output. The sources can be directly modulated lasers,
or continuous wave lasers together with separate modulator
elements, for example. The sources can also incorporate drivers
with the modulators or with the lasers. The light from these
multiple sources, each generally at a different wavelength, are
generally coupled to a chip that multiplexes the light from all the
sources into a single output.
This is schematically shown in FIG. 1 for a 4.times.10 Gb/s
transmitter. The transmitter includes a laser diode chip 10 with a
laser that sends a beam of light forward into a microlens 20 that
in turn focuses the beam into a planar lightwave circuit (PLC) 40.
In some embodiments the laser output wavelength shift is about 0.1
nm per degree Centigrade. Behind the laser is a driver 30, which
provides electrical signals to the laser diode chip. Note that
there are four sets of lasers, drivers, and microlenses on the
assembly of FIG. 1.
In some embodiments the lasers are distributed feedback (DFB)
lasers. In various embodiments the lasers are of an InP based
material, and may be for example of AlGaInAs/Inp or InGaAsP/InP. In
some embodiments the microlenses are mounted on moveable arms, for
example as discussed in U.S. Patent Application Publication No.
2012/0195551, entitled MEMs Based Levers and Their Use for
Alignment of Optical Elements, and U.S. Patent Application
Publication No. 2011/0013869, entitled Micromechanically Aligned
Optical Assembly, the disclosures of which are incorporated by
reference herein.
The PLC that muxes the light together is generally designed to have
a passband wavelength dependence with respect to temperature which
is the same as or substantially the same as that of the lasers. In
some embodiments the temperature dependence may be half that of the
lasers, or between half that of the lasers and the same as the
lasers, or within 30% of that of the lasers. In many embodiments
the PLC is made of glass, for example silica based, incorporating
grooves filled with a polymer material in waveguides of the PLC. In
some embodiments the polymer material is a silicone resin. In some
embodiments the passband wavelength shift is about 0.1 nm per
degree Centigrade. The typical operating temperature of the
assembly is from -5 C to 75 C, and thus over 80 C temperature
difference, one sees about an 8 nm shift in passband wavelength. As
the PLC has a much greater passband wavelength shift with respect
to temperature than would otherwise be expected, the PLC may be
considered a "superthermal" device.
In some embodiments the superthermal device includes a groove
structure filled with a material with change in refractive index
with respect to temperature (dn/dT) different than the dn/dT of the
core of the PLC. Preferably the dn/dT of the material is either a
highly positive or highly negative dn/dT, such as the dn/dT for a
silicone resin. This, for example, allows the PLC, for example an
arrayed waveguide grating (AWG), to have a higher temperature
dependent passband center wavelength shift that is much more
closely matched to that of an active device, for example such as a
semiconductor laser. By varying the groove geometries, devices with
arbitrary d.lamda./dT can be achieved on the same silica platform.
Integration with devices that have matched d.lamda./dT gives
advantages of eliminating, in many cases, heating/cooling elements
within the integrated module without compromising AWG design and
performance.
FIG. 2 shows a further embodiment in accordance with aspects of the
invention. The embodiment of FIG. 2 includes a plurality of
semiconductor lasers 10, for example DFB lasers. Light from each of
the lasers is focused by a corresponding lens 25 into a
corresponding input of an AWG 45. The lasers and the AWG are
coupled to a common substrate, with intervening substrates present
in some embodiments.
The AWG has a section 42, triangular in some embodiments, made in
the waveguide arms that are etched out of a region of the glass in
which gratings of the AWG are formed, and replaced with polymeric
material with a different dn/dT than the dn/dT of the glass. In
most embodiments the polymeric material, and the amount of
polymeric material in each waveguide of the AWG, is selected such
that the AWG is a superthermal AWG. In some embodiments the
polymeric material, and its amount, are selected such that the
dn/dT of the AWG matches the variation in output wavelengths with
respect to temperature of the lasers. To reduce diffraction loss,
in some embodiments the groove is replaced with divided grooves,
for example in the form of multiple grooves, which does not allow
the light to diffract considerably while in the unguided polymer.
In various embodiments the waveguides may be widened to reduce
loss. Though such configurations sometimes affect the polarization
response of the AWG, this is generally not important for a
multiplexer that operates only on a single polarization.
In general the wavelength sensitivity of a superthermal AWG is
determined by the dn/dT coefficients of the waveguide and the
groove filling material. The center wavelength (.lamda..sub.e) of
the passband of the AWG is determined by the equation below:
.lamda..sub.c=.DELTA.L.sub.c/m*n.sub.c*(1+n.sub.p*.DELTA.L.sub.p/(n.sub.c-
*.DELTA.L.sub.c)) (1)
where .DELTA.L.sub.c is the length difference between each adjacent
grating in the silica waveguide, n.sub.c is the effective
refractive index of the silica grating waveguide, .DELTA.L.sub.p is
the length difference between each groove length for adjacent
grating waveguide regions, n.sub.p is the index of refraction the
groove filling material, and m is the grating order.
The temperature dependence of center wavelength is given below:
d.lamda..sub.c/dT=1/m*(dn.sub.c/dT*.DELTA.L.sub.c+dn.sub.p/dT*.DELTA.L.su-
b.p) (2)
where typical dn/dT values (ignoring second order temperature
dependence) are: dn.sub.c/dT=1.1.times.10.sup.-5/.degree. C., and
dn.sub.p/dT=-37.times.10.sup.-5/.degree. C.
Combining eq(1) and eq(2),
.DELTA.L.sub.p=(m*d.lamda..sub.c/dT-dn.sub.c/dT*.DELTA.L.sub.c)/(dn.sub.p-
/dT) (3)
.DELTA.L.sub.c=m*.lamda..sub.c/n.sub.c*{[1-(d.lamda..sub.c/dT)/.-
lamda..sub.c*n.sub.p*(dn.sub.p/dT)]/[1-(n.sub.p/n.sub.c)*(dn.sub.c/dT)/(dn-
.sub.p/dT)] (4)
By selecting an appropriate .DELTA.L.sub.p value, the AWG can be
made to have a d.lamda..sub.c/dT that matches that of other devices
like semiconductor lasers that have approximately 10 times the
temperature sensitivity.
For example, for a 10 channel 400 GHz spacing AWG to match a
d.lamda./dT of approximately 100 pm/.degree. C. of a laser,
equations (3) and (4) may be used to calculate .DELTA.L.sub.p to be
-7.66 um (assuming a nominal center wavelength of 1.55 um, n.sub.c
of 1.4561, n.sub.p=1.4, and m of 32).
A design of an AWG based on the above is shown in FIG. 3. The AWG
has 10 channels with 400 GHz spacing, and the refractive index
contrast is 1.5% with a core geometry of 4 um.times.3.5 um. As one
can see, .DELTA.L.sub.p is a negative number, with a triangular
region 61 decreasing in width from a bottom, shorter, waveguide 63
to a top, longer, waveguide 67. To cover the 65 grating waveguides
in the device, the bottom grating waveguide should have an extra
length of the silicone region of 498 um compared to that of the top
waveguide. As an easy way to implement this, straight waveguides of
equal length can be inserted in the middle of the grating region of
the AWG to accommodate a rectangular shaped etched trench that is
filled with silicone. However, diffraction loss resulted from a
long unguided silicone region in a single rectangle of this size
may be unacceptable in many cases. Divided grooves 71 instead of a
single groove 73 may be implemented to reduce diffraction loss. In
this case, dividing the single rectangle into 100 equally spaced
narrower rectangles, such that each silicone filled region is no
more than Sum long along each grating waveguide, could improve the
insertion loss of the device significantly.
FIG. 4 is a graph showing an example Gaussian passband response of
a channel of a superthermal AWG in accordance with aspects of the
invention. A first curve 140 shows an example Gaussian response of
a superthermal AWG, while a second curve 100 shows an example
flat-top passband response. As may be seen through a comparison of
the two curves, Gaussian response has a higher peak, but passes
light in a narrower range of wavelengths.
FIG. 5 shows the optical characteristics of such as system, where
Gaussian passbands are used that shift with temperature. 140 is the
Gaussian passband curve of the first filter while 110 is the laser
wavelength matched to that filter at low temperature. Once the
temperature increases, the laser wavelength moves to 130, but the
filter response moves the same amount to 150. The match between the
laser and the filter is maintained. Since the Gaussian filter has
much lower loss than the flattop, the efficiency of the module is
increased and the laser can run at lower power, saving power
consumption.
Commensurately, one can increase the number of channels of this
uncooled system and space them closer together. All the channels
will drift up and down with temperature together, and one can use a
demultiplexer to track this drift and appropriately lock on to the
grid. This can be done in many ways. For example, the receiver can
be made tunable by controlling the temperature of the
demultiplexer. Since the demultiplexer does not generate heat, it
can be thermally insulated from the environment and therefore only
a small amount of power from a heater would vary the temperature
substantially. This would tune the filter. This heater could be
made local--for example on a polymer insert into the PLC, or it
could heat the entire assembly. To track, a low frequency dither
tone can be placed on one channel of the transmitter. The receiver
would then detect this dither tone, and adjust the temperature of
the receiver with heater power such that the dither would be
maximized at the appropriate channel.
The region of different index can also be implemented in areas of
the PLC other than the grating waveguides, for example the star
region. FIGS. 6 and 7 show implementations where part of the slab
of the PLC is etched out and replaced with polymer. The
implementation is similar to the version where the grooves are in
the grating region, except that the grooves here are concentrically
shaped with respect to the center of the input slab region 81, so
that the light in the slab region enters the silicone filled
grooves at or close to normal. The effect is that same in that the
beam is steered with temperature causing the center wavelength of
the multiplexer to shift much more dramatically with
temperature.
Although the invention has been discussed with respect to various
embodiments, it should be recognized that the invention comprises
the novel and non-obvious claims supported by this disclosure.
* * * * *